This invention relates in general to alerting devices employed in a system for alerting one or some of a large number of persons from a central paging station. In practice, for example, a caller of such a person may reach the central station by telephone, and the paging station may signal to the person via a radio common carrier (RCC) link. The invention thus relates more particularly, by way of example, to personal paging devices which can be carried on the person of a user, for response to space-transmitted calling signals such as radio waves. Alerting signals provided by such devices may be audible, visible or tactual (e.g.: vibratory), and alerting devices are known which give the user a choice of one or more alerting modes. Whatever alerting mode or modes are employed in a particular alerting device, the user must be wearing the alerting device, or at least be sufficiently close to it to perceive the alerting signal, in order for the alerting device to accomplish its purpose. This invention is addressed to situations in which a user of an alerting device which is energized to receive calling signals nevertheless fails for one reason or another to perceive an alerting signal that is provided by the device in response to a calling signal.
In the art of paging persons by means of portable alerting devices, such as radio receivers carried on the person that are responsive to an assigned carrier frequency, it is known to modulate the carrier with a sequence of calling frequencies, or "tones," for the purpose of signalling to subscribers with unique combinations of tones, for example, to address a particular subscriber, or to broadcast a particular message. A known two-tone paging system uses two tones selected in a coded sequence from an array of available tones. In another system, five tones are sent out (i.e.: modulate the carrier) in a coded sequence, but the number of tones used in a given coded sequence is not critical. In any system, each individual receiver is responsive to a selected one or few of all the possible useful codes. In a five tone paging system which is in common use, each tone last 33 milliseconds, and there is a 35 millisecond gap between pages, resulting in (5 .times. 33) + 35 = 200 milliseconds for a complete page. Such a 5-tone paging system allows five pages per second. Thus, an alerting device in such a system has one-fifth of a second to receive a calling or paging signal and to provide an alerting signal in response to it. The alerting signal can, however, have a duration which is independent of the duration of the paging signal.
It is known according to U.S. Pat. No. 3,742,481 to disable an audio calling device in a radio pager, and to store the calling information so that it can be reproduced when desired. In the system of that patent this feature is used when the user does not wish to be disturbed or to cause a disturbance when the pager is alerted. When this feature is used, the pager is on, but will not emit an alerting signal. When the need for silence has passed, the pager is reset, and the alerting signal is furnished if a call has been received during the silent period. A practical defect of that system is that the user may forget to reset the pager to the condition for emitting an alerting signal in response to a calling signal, and when that happens the alerting device becomes useless, in a practical sense. The purpose of establishing a RCC link, which is to reach its subscribers with a minimum of delay, is thereby frustrated, and a user who forgets to reset a pager from the stored information mode to the alerting mode might just as well limit his or her interest to a telephone answering service. Understandably, operators of RCC Paging Networks and those offering similar services prefer not to supply their subscribers with devices that have the capability to frustrate the service intended to be rendered, and there is a requirement for an alerting device which is more reliable from a system-servive point of view.